Pages

Wednesday, November 27, 2013

Digestive System, Part 9: The Colon

Adapted from an image CC CC Olek Remesz.

The last time we looked at the digestive system, feces was piling up in the cecum. So where’s it going from there?

We’ve already gone over contractions in the colon that serve primarily to mix, but the real movers and shakers in the colonic mobility world are Giant Migrating Contractions. GMCs are real truth in advertising: contractions that cause giant migrations (of poop.) They occur in the small intestines too, to move chyme into the cecum, but in the colon they’re notable for occurring only three times per day, and moving feces a full third of the way through the colon at one go.1 You’ve probably noticed these from time to time, as a sensation of sudden movement inside your belly. The next time you notice one, make sure to tell the person next to you that your colonic feces has just undergone a Giant Migrating Contraction. If they don’t edge away from you, it’s love.

The colon has a few jobs to do, but what it's really good at is water absorption. Although the small intestine absorbs more water in a pure liter for liter sense, its job of absorbing water from soaking-wet chyme is actually pretty easy. If you imagine your intestinal contents as a sopping sponge, the small intestines give it a light squeeze to get most of the water out, but it takes the colon to really wring that sucker dry. Speaking in terms of percentages, the small intestine absorbs the first 81% of the water that enters it. The colon takes care of another 16%, leaving less than 3% of the original water behind and transforming the feces from a watery goop into a solid.2

If that last 16% doesn't seem very important, remember that it takes a lot of water to digest your food. On an average day, you may consume two liters of water, but a full nine liters of water passes through the gastrointestinal tract. The remaining seven liters come from saliva, stomach secretions, bile, and digestive juices.3 So, without the colon to wring out those last drops, you’d be losing over a liter of water per day just to digestion, in addition to what you normally lose in sweat and urine. To make that up, you’d have to drink more (and absorb it with your less-efficient small intestines.) All else being equal, if you didn't have a colon, you’d need to drink twice as much to survive.

So how do the intestines absorb water? They take advantage of a fun little trait of osmosis: water likes to migrate toward areas with high concentrations of dissolved material. The enterocytes in the small and large intestine take advantage of this by pumping sodium ions into the spaces between cells. The trick here is that, once it's been pumped in, sodium can’t easily get back into the enterocytes, but water can pass back and forth with little effort. This creates an “osmotic gradient”. Water rushes into the spaces (i.e. down the gradient) to join the sodium and restore the balance of the universe, which those pesky enterocytes have thrown out of whack. Water and sodium then both diffuse out of those intercellular spaces and into the bloodstream. Badda bing, badda bang, you’ve absorbed water and picked up an electrolyte for free. The colon is better at this than the small intestine, because it can collect and pump sodium ions faster.4

Yes, I drew those diagrams myself. How did you know? Because they're so awesome, right?

Other electrolytes are pumped back and forth across the enterocytes' membranes, cotransported, or simply allowed to diffuse passively into the bloodstream. All this electrolyte transport is old hat to the human organism. Maintaining proper electrolyte balance is one of the most basic functions required for life, so some of these electrolyte transporters have been with us almost from the beginning, evolving billions of years ago and being preserved and modified through the eons, to eventually be passed down to us.5 It's like a family tradition that started with your great grandma and was handed down through the generations, except in this case it started with your great-times-several-billions grandma, who was a prokaryote.

Although it’s not highly adapted for it, the colon is also capable of limited absorption of non-electrolyte nutrients. Some of these nutrients (such as vitamin K and B7) can be produced by your gut bacteria, so there's an advantage in being able to absorb them where the bacteria live.6 In humans, the products of these bacteria are of small utility, but some animals find them more critical.

For example, non-ruminants who consume large quantities of indigestible plant fiber (such as horses, elephants, some rodents, and wabbits) often host vital, plant-eating bacterial colonies in their colon and/or cecum. These bacterial colonies turn the large intestine into a fermentation chamber, breaking plant cellulose into digestible starches and sugars that can be absorbed in the colon.

But the colon isn’t as good as the small intestine at absorbing nutrients, so ruminants (animals such as cows, with bacterial colonies living high in their digestive tract, in specialized stomach compartments) are more efficient at getting energy from plant fiber.7 Suck on that, horses and elephants.

In humans, the colon runs for about five feet. The magical, disgusting journey through its depths takes us up through the ascending colon, across the transverse (or “lying across”) colon, down the descending colon, and then into an s-shaped turn called the sigmoid (or “s-shaped”) colon. Yeah, really fucking creative names, guys. I think the committee for naming anatomical stuff was about ready to go home when they got this deep into the digestive tract.

Speaking of which, our feces is in the home stretch now. Next time, we'll take a quick detour to the appendix, and then it's out through the rectum! What fun!

**

If for some reason you've actually read this far, you'll definitely want to check out the other articles in this series: